A mutual savings bank may invest not to exceed five percent of its funds in the bonds of any irrigation, diking, drainage, diking improvement, or drainage improvement district of this state, unless the total indebtedness of the district after the completion of the improvement for which the bonds are issued, plus the amount of all other assessments of a local or special nature against the land assessed or liable to be assessed to pay the bonds, exceeds forty percent of the value of the benefited property, exclusive of improvements, at the time the bonds are purchased or taken by the bank, according to the actual valuation last placed upon the property for general taxation.

Before any such bonds are purchased or taken as security the condition of the district's affairs shall be ascertained and the property of the district examined by at least two members of the board of investment of the mutual savings bank, who shall report in writing their findings and recommendations; and no bonds shall be taken unless such report is favorable, nor unless the executive committee of the board of trustees after careful investigation is satisfied of the validity of the bonds and of the sufficiency of the assessment or other means provided for payment thereof: PROVIDED, That no mutual savings bank shall invest a sum greater than three percent of its funds, or, in any event, more than three hundred thousand dollars, in the bonds of any one district described in this section.